Added desktop & mobile views to welcome email editor (TryGhost#26495)

ref https://linear.app/ghost/issue/NY-1056

*I recommend [reviewing this with whitespace changes
disabled](https://github.com/TryGhost/Ghost/pull/26495/changes?w=1).*

Gated behind a feature flag, the welcome email editor now has a desktop
and mobile view.

With the feature flag off:



![Before](https://github.com/user-attachments/assets/81fda0a9-ba57-43fe-9f5c-41b8ece18d40)

With the feature flag on:

![After
(1)](https://github.com/user-attachments/assets/cfa22c65-e350-42cb-8da5-f3aa5ae57496)
![After
(2)](https://github.com/user-attachments/assets/92c62e35-e275-4a38-be3a-fa585ba1ddab)

---------

Co-authored-by: Steve Larson <9larsons@gmail.com>

Author: EvanHahn